Referring to FIG. 1, FIG. 1 is a schematic cross-sectional view illustrating a conventional liquid crystal display. At present, an AC voltage is used to drive an LCD, therefore there are two electrodes respectively disposed on both sides of a liquid crystal layer, one being a common electrode 100 and another being a pixel electrode 200. There exist positive or negative impurity ions 10 in the liquid crystals.
Referring to FIG. 2, FIG. 2 is a schematic drawing illustrating a waveform of conventional AC driving voltages. During a fixed image, two different pixel voltages Vp1 and Vp2 are provided for the pixel electrode 200 in different frames, this enables the common electrode 100 (the voltage thereof is a VCOM) and the pixel electrode 200 to form electric fields with opposite directions and identical voltage differences, thereby driving the LCD in the AC voltages. Referring to FIGS. 3 and 4, FIG. 3 is a schematic drawing illustrating the impurity ions moving when Vp1 is added; and FIG. 4 is a schematic drawing illustrating the impurity ions moving when Vp2 is added. When adding the voltage Vp1, that is, Vp1<VCOM, the distances which the positive or negative impurity ions 10 move are labeled d. When adding the voltage Vp2, that is, Vp2>VCOM, the opposite direction which the positive or negative impurity ions 10 move are also labeled d. Therefore, the impurity ions 10 are not gathered collectively.
In general, the voltage of the common electrode 100 is fixed. However, in order to achieve the image quality improvements or implement other driving architectures, the voltage of the common electrode is set to be changeable. Either way, the absolute values of the voltage differences between two ends of the electrodes are as equal as possible. Referring to FIGS. 5 and 6, FIG. 5 is a schematic drawing illustrating a waveform of the driving voltages with two unequal voltage differences; and FIG. 6 is a schematic drawing illustrating the moving impurity ions when adding the voltages shown in FIG. 5. While the absolute value of the difference between Vp1 and VCOM is unequal to the absolute value of the difference between Vp2 and VCOM, the moving distances d1 and d2 of the positive or negative impurity ions are not the same. This will result in charge residuals, that is, the impurity ions within the liquid crystal layer will gather collectively on both sides, as shown in FIG. 6.
When more of positive and negative impurity ions 10 are attached to the common electrode 100 and an alignment film (not shown) of the pixel electrode 200, an internal voltage Vi can be formed. Accordingly, when Vp1 and Vp2, which correspond to a certain gray scale, are provided, the differences between Vp1 and Vp2 in relations to VCOM are affected by Vi, and then tilted angles of the liquid crystal molecules are changed. Thus, flicker and color deviation may occur on the images. Moreover, the attached impurity ions will always be attached to the alignment film and cannot be restored. So the above-mentioned non-recoverable and non-maintenance occurrence is referred to as a liquid crystal polarization problem.
Therefore, in order to ensure that the liquid crystals are not polarized, there is a need for a stable common electrode voltage to form the same absolute values of the differences between the common electrode voltage and the pixel voltage. However, as for the whole display panel, it is difficult to make the voltage on every common electrode to be same. At present, all the conventional techniques intend to solve the above-mentioned problem is by reducing the charge residuals, but the problem still occurs after a long operation time.